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Turning my Zone 5 attic into conditioned space

user-1107451 | Posted in Energy Efficiency and Durability on

So have been scouring for iso to do my basement walls and needed only 40 sheets, but Ive come across a guy who has 100 sheets and needs to get rid of them all at a crazy price.
So my question is can I use this 3′ iso on the underside of my rafters to bring my attic into a conditioned space.
The attic insulation is already at R60 (R38 batts and the rest blown in cellulose) but I thought if I have all this left over Iso I may as well use it in the roof and give my central air a break.

It has soffets and a roof ridge vent so my plan was to run the iso from where the rafters meet the attic floor up to say 6ins from the roof vent so the sheathing can breath, and do the ends up to the ridge.

Does that sound right I will tape all seams as I go but have I missed anything?

Education is always appreciated


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  1. GBA Editor
    Martin Holladay | | #1

    It's too bad that you went ahead and installed R-60 insulation on your attic floor if you have an air handler and ductwork in your attic. The money spent on that attic floor insulation would have been better invested in creating an unvented conditioned attic (by installing insulation that follows your roof plane).

    If you want to install your extra polyisocyanurate on the underside of your rafters, you can. The insulation will help keep your attic cool. If you want to maintain ventilation through your rafter bays -- and I think that it's a good idea if you do -- then you need to verify that the soffit vents are unclogged and your ridge vent is unclogged.

    Once you've done that, you want to install the polyiso in an airtight manner, all the way up to the ridge. The ventilation occurs on the exterior side of the polyiso. The polyiso should be sealed, even at the ridge. The polyiso on one side of your gable roof -- let's call it your south slope -- should meet the polyiso on the north slope with an airtight (taped) seam at the ridge.

    For more information on this topic, see Creating a Conditioned Attic.

  2. user-1107451 | | #2

    The house came with the R38 and the air handler so I air sealed and added the cellulose (Thank you Martin for the advice, and that cut my power bill in half.
    I checked the vents were open when I put the cellulose in
    So Im glad you corrected me on the roof ridge I wasnt 100% sure but thanks for correcting me

  3. Expert Member
    Dana Dorsett | | #3

    With R60 on the attic floor and ~R15 (derated for climate zone) on the rafters there is a potential dew-point control issue in the attic space in winter in a zone 5 attic, but it's complicated by the fact that the floor insulation isn't in contact with the rafter insulation.

    In zone 5 you need 40% of the total-R to be on the exterior of the fiber insulation for dew point control, and you would have only about 20%. That means the average wintertime temp in that attic space may be considerably below the dew point of your conditioned space air. Any duct leakage would only aggravate that problem.

    But since the R60 is partially thermally bridged by joists, the total performance of the attic-floor layer is probably more like R45-R50, better, but still insufficient margin to be a slam-dunk. Then considering there is more roof area than attic floor area, it worsens again ... (sigh).

    Bottom line- don't do it. The roof deck would be fine as long as there is adequate soffit-to-ridge venting, but the risk of mold taking off in the attic is too high. If you want to use the extra polyiso to build out covers over the air-handler & ducts, air-sealing the polyiso to the ceiling below, then blowing cellulose over it you'd get some thermal improvement without the mold risk, but that's a different level of project.

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